Multi-resistant Gram-negative bacilli (MRGN)

Gram-negative bacilli are a large group of bacteria that are commonly found in the intestinal tract of humans and most animals. They form part of the normal microflora and are essential for proper digestive processes. However, these bacteria are capable of causing infection when introduced into normally sterile body sites, such as the bladder or deep tissues, particularly via insertion of a medical device or during surgery. Serious infections require the administration of antibiotics and can be associated with a high mortality rate, particularly in vulnerable patients such as those in critical care or who are immune-suppressed.

Antibiotic resistant strains

Over the past few decades concern has been expressed about the increasing incidence of resistance in this group of bacteria, making serious infections with these organisms increasingly difficult to treat. This is a worldwide problem that requires a global response, and there are many campaigns to raise awareness of the importance of restricting antibiotic use in order to preserve their effectiveness. This strategy is known as antimicrobial stewardship.

Types of resistance

Resistance in Gram-negative bacilli occurs by several mechanisms and there are literally dozens of different resistance determinants. Those that pose particular problems for hospital patients can be broadly grouped into:

• transferable beta-lactam resistance, for example AMP-C, mostly found in E.coli and Salmonella species
• extended-spectrum beta-lactamases (ESBL) (PDF 73KB) – transferable resistance to 3^rd and 4^th generation cephalosporins, mostly found in E.coli, Klebsiella and Enterobacter species
• metallo-beta-lactamases (MBL) – similar to ESBL, but can also include resistance to carbapenems, mostly found in Pseudomonas aeruginosa
• carbapenemase-producing Enterobacterales (formerly known as Carbapenemase-producing Enterobacteriaceae), an emerging resistance of concern.

The percentage of Gram-negative bacilli identified as carrying one or more of these resistance determinants is still relatively low in Australia compared to Asian countries and parts of Europe and the USA. Returned travellers are becoming a significant source of asymptomatic carriage, therefore a recent travel history on admission is important to detect potential risk.

Mode of transmission

Since these organisms are mainly found in the intestine, any environmental surfaces that come into contact with faecal material can become contaminated and serve as a reservoir for cross-infection. In addition, these organisms generally prefer a wet environment, and can quickly colonise sink drains and taps. They have also been known to contaminate diluted disinfectants and detergent solutions used for cleaning. The primary mode of cross-transmission is via the unwashed hands of clinical staff. Patients may also spread infection by touching their own urinary catheter or wound drainage tube. The duration of asymptomatic carriage of MRGN in the bowel varies considerably, but may be months to years in low numbers.

Prevention

The prevention of infection with MRGN involves the simultaneous application of a number of strategies:

• identification of carriage (screening of high risk patient groups) and appropriate inpatient placement for example single room for patients that pose a high risk of transmission
• diligent hand hygiene practices, including alcohol-based hand rubs at point of care
• application of personal protective equipment when performing patient care activities
• appropriate cleaning and disinfection of patient surroundings and medical equipment
• antimicrobial stewardship – making sure that antibiotics are prescribed appropriately.

Management

All patients infected or colonised with MRGN should be managed with transmission-based precautions. Hospitals should have a strategy for management of MRGN infection with practices adapted to suit the clinical setting. For example, management of patients identified with MRGN in intensive care may be different to management of a patient attending an outpatient department, where the risk of transmission and infection are much lower.

Further information

For further information on the management of patients with MRGN contact SA Health’s Infection Control Service.